The U.S. Ambassador to South Korea received 80 stitches in his face after he was attacked by a knife-wielding assailant Thursday morning in Seoul.

The attacker is an ultra-nationalist political activist who claims he acted alone.

Ambassador Mark Lippert, 42, was listed in stable condition Thursday after undergoing surgery at the Sinchon Severance Hospital in Seoul to repair lacerations to his face and left hand. The hospital director Jung Nam-shik said the ambassador needed stitches to close the deep gash to his face.

Jung said the gash on Lippert's face was about 11 centimeters (4 inches) long and 3 centimeters (1 inch) deep, located from his right cheekbone to jaw. He added that there was no major damage to Lippert's facial nerves or salivary glands.

Doctors also treated his thumb and two fingers on left hand that were cut during the attack. The Associated Press reported that surgery was performed to repair tendon and nerve damage.

The attack happened about 7:40 a.m. local time Thursday (2240 GMT Wednesday).

Later Thursday, Lippert wrote on his Twitter account that he is doing well and is in great spirits.

U.S. President Barack Obama spoke with Lippert after the attack, wishing him a speedy recovery, said White House national security spokeswoman Bernadette Meehan.

The U.S. State Department "strongly condemns" the attack, spokeswoman Marie Harf said in a statement. "The U.S.-ROK alliance is strong; we will not be deterred by senseless acts of violence. "

The State Department also said it could not speculate on a motive for the attack at this time.

U.S. Secretary of State John Kerry told the AP on Thursday that Lippert was "doing as good as can be expected" after he was attacked in South Korea.

Kerry, who was in Saudi Arabia for meetings with Gulf state officials, said, "The United States of America will never be intimidated or deterred by threats or by anybody who harms any American diplomats."

Democratic Senator Patrick Leahy said in a statement that he was "heartsick ... about the senseless attack on Mark. ... He is as devoted a public servant as they come, a great credit to both of our countries." Lippert served as a former policy aide to Leahy.

South Korean President Park Geun-hye said the incident was "not only a physical attack on the U.S. ambassador in South Korea but also an attack on the Korea-U.S. alliance and we will not tolerate it."

South Korea's Foreign Ministry condemned the attack and vowed a thorough investigation and strengthened protection of embassies.

The attack

Lippert was attacked by a Korean assailant with a 25-centimeter-long (10 inch) knife, described as a fruit knife, while attending a forum on Korean reconciliation. The organization that hosted the event, the Korean Council for Reconciliation and Cooperation, is a coalition of various groups that support the peaceful unification of the two Koreas.

Lee Sang-heon, the group’s director, denied reports the assailant, Kim Ki-jong, 55, was a representative from any of the member groups.

Lee said Kim was able to gain admittance by taking advantage of a bureaucratic oversight, saying he was from an old group that no longer exists but remains on the organization’s list.

A man, bottom center, who attacked U.S. ambassador to South Korea Mark Lippert, is detained by police at the Sejong Cultural Institute in Seoul, March 5, 2015.

Lee apologized for the breach of security but said they did not think there was any cause for broader concern.

He said it was an annual event and there had been no such incident like this in past, so they did not request extra police presence.

Michael Lammbrau of the Arirang Institute think-tank was at the meeting and told Reuters, "The guy comes in. ... He yells something, goes up to the ambassador and slashes him in the face.”

Lammbrau said Kim shouted about Korean independence while he was being restrained. "It sounded like he was anti-American, anti-imperialist, that kind of stuff," he said.

"The ambassador fought him from his seat. ... There was a trail of blood behind him," Lammbrau told Reuters.

U.S. ambassadors have security details, but the size largely depends on the threat level of the post, the AP reported.

Security detail

Seoul is not considered to be a particularly high threat post despite its proximity to the North Korean border. It's not clear how many guards Lippert had, but they would have been fewer than the ambassadors in most of the Mideast, the AP reported.

Kim reportedly told police he acted alone. When he attacked the ambassador, he reportedly shouted, "South and North Korea should be reunified!”

In a recent blog post believed to have been written by Kim, he blamed the joint U.S.-South Korea military exercises, which began Monday, for preventing the reunions of families that have been separated since the end of the Korean War in 1953. North Korea recently cited the joint drills as the reason why they would not permit the reunions to take place.

On Thursday, Pyongyang said the attack on the ambassador was "just punishment" for the U.S. going ahead with the joint military drills.

Past incidents

South Korean media reports that Kim has a long history of ultra-nationalist activism and violence that includes burning a U.S. flag on the embassy grounds in Seoul in 1985, staging one-man protests against Japan over disputed islands, and attempting to light himself on fire during a protest in front of the presidential Blue House in 2007.

Seoul's Foreign Ministry said it was the first time a foreign ambassador stationed in modern South Korea had been injured in a violent attack. But police said Kim was arrested in 2010 for throwing concrete stones at the Japanese ambassador and received a suspended two-year jail sentence.

Kim is well-known among police and activists as one of a hard-core group of protesters willing to use violence to highlight their causes, the AP reported. Such protesters often portray their causes as matters of life and death.

There are also reports alleging Kim visited North Korea six times between 2006 and 2007 and that he tried to erect a memorial in Seoul for the late North Korean leader Kim Jong Il in 2011.

South Korean officials condemned the attack, calling it an unacceptable harmful act and a very serious crime against the envoy representing South Korea’s most important ally.

The South Korean Defense ministry said the attack will not affect the ongoing joint military exercises, which involve thousands of troops from the United States and South Korea and are meant to prepare for a possible attack from North Korea. The exercises continue for eight weeks.

Before he became ambassador in Seoul in October 2014, Lippert was a U.S. Senate aide to Obama and served in the U.S. Navy in Afghanistan and Iraq, winning the Bronze Star. He was chief of staff for former Defense Secretary Chuck Hagel before taking up his post in Seoul.

Lippert has been a regular presence on social media and in speeches and presentations during his time in the South Korean capital, the AP reported.

VOA News Producer Youmi Kim contributed to this report. Cindy Saine contributed to this report from Washington. Some material for this report came from Reuters and AP.